A new focal immunofluorescence assay (FIA) was developed for use in assaying and biologically cloning retroviruses infecting live cell monolayers. By using monoclonal antibodies with specificity for only certain strains of murine retrovirus, it was possible to measure interference to superinfection of chronically infected cells. Surprisingly the patterns of interference varied markedly when the same viruses were compared in different cell lines. These results suggested that specificity of cellular receptors for retroviruses could vary widely in different cell lines. Furthermore, the patterns of interference seen in one particular cell line could not be explained by the presence of a small number of different receptor types. The FIA has also been used to compare virus replication and recombinant MCF virus generation in mice inoculated with a low virulence variant Friend murine leukemia virus strain (F-MuLV-B3). Results indicated that B3 was not defective in replication or recombinant MCF virus generation, nevertheless in vivo leukemic transformation as detected by splenomegaly or lymphadenopathy was delayed by several months compared to the parental virus strain (F-MuLV 57). Preliminary results suggest that strains B3 and 57 may not differ in ability to transform hemopoietic cells, but rather in their ability to induce hemolytic anemia with compensatory splenomegaly early after inoculation.